In recent years numerical methods for the analysis of underground structures have advanced rapidly, creating a sophisticated array of mathematical tools for the design and evaluation of structures such as tunnels, mine structures, underground openings building foundations, dams and other large civil engineering projects, and the like. To fully exploit the precision and power of these mathematical methods, it is necessary to provide accurate input data to their computer programs regarding the stress state and material properties of the earthen media which will host the underground structure. Unfortunately, the development of instruments for acquiring the required in situ data has lagged far behind the numerical methods and the software that generally embodies these methods. Furthermore, even if the required data had been obtained, there is still no reliable means to examine the validity of the outcome of such numerical analysis. Thus mining and civil engineering design are hampered by a lack of reliable, precise data.
Conventional methods for measuring the needed in situ stress state of underground media include overcoring, hydrofracturing, core relaxation, borehole slotting, and related techniques. Overcoring is practical only in earthen media that is close to a (theoretically) idealized state, which is seldom found in the real world, and hydrofracturing is applicable only in uniform, isotropic non-fractured ground. All the other stress measurement methods are found to be not very useful in practice. Instruments such as a presiometer or Goodman jack are designed only to measure material properties, but not stress states. At present, therefore, there is no instrument which is capable of measuring both stress states and material properties simultaneously. To measure both, a combination of techniques must be used, an approach that can be burdensome and synergistically inaccurate. None of these approaches provides an opportunity for continuous monitoring or periodic measurement of stress state and material properties in underground media, and changes in stress state and material properties may be critical in early detection of catastrophic events such as rock bursting, opening deterioration, mine failure, earthquake, landslide, or the like.
The state of the art in instruments for measuring material properties and stress state in earthen media is described in U.S. Pat. No. 4,733,567 to Serata. This device includes a sealed plastic cylinder placed in a borehole and inflatable by hydraulic pressure to expand uniformly against the borehole wall. A plurality of LVDT sensors are arrayed diametrically within the cylinder to detect fracturing of the borehole. The expansion pressure is increased until initial fracturing is achieved, indicating that the combined tensile strength of the media and the ambient stress have been exceeded. By deflating and then repeating the process, the tensile strength and the principle stress vectors may be resolved. This approach is effective in homogenous media under certain restricted stress states, but is less successful in media having non-uniformities discontinuities, microfractures or prefractures, or viscoplastic characteristics. Also, it is not applicable to continuous automated monitoring and recording of underground stress states.
Thus the prior art lacks an effective technique and instrument for simultaneously providing accurate and reliable data on stress states and material properties, and it is not possible to take full advantage of the powerful numerical methods now available for analysis, design, and safety assurance of underground structures.